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Involvement of both major histocompatibility complex class II alpha and beta chains in CD4 function indicates a role
1Sealy Center for Molecular Science, University of Texas Medical Branch, Galveston 77555-1019, USA.
Insights
CD4 glycoprotein on T cells stabilizes T cell receptor interactions with peptide:MHC class II. Mutations reveal a second critical binding site on MHC class II, highlighting the role of CD4 and MHC class II oligomers in T cell activation.
Area of Science:
- Immunology
- Molecular Biology
- Cell Biology
Background:
- CD4 is a crucial membrane glycoprotein on T lymphocytes.
- It interacts with peptide:major histocompatibility complex (MHC) class II molecules, stabilizing T cell receptor (TCR) binding.
- This interaction is vital for T cell activation and signaling.
Purpose of the Study:
- To identify and characterize additional binding sites for CD4 on MHC class II molecules.
- To investigate the functional significance of these newly identified sites in CD4 coreceptor activity.
- To elucidate the role of CD4 and MHC class II oligomerization in T cell activation.
Main Methods:
- Site-directed mutagenesis was used to introduce mutations in specific regions of the MHC class II molecule.
- Functional assays were performed to assess the impact of these mutations on CD4 binding and coreceptor activity.
- Analysis of T cell activation was conducted in response to altered MHC class II-CD4 interactions.
Main Results:
- A previously unknown surface-exposed segment in the alpha 2 domain of MHC class II was identified as critical for CD4 function.
- Mutations in this alpha 2 domain site, similar to mutations in the previously known beta 2 domain site, diminished CD4 coreceptor activity.
- These two critical sites are spatially arranged such that they cannot interact with the same CD4 molecule simultaneously.
Conclusions:
- The findings indicate that CD4 binding to MHC class II involves at least two distinct sites.
- The spatial arrangement of these sites suggests that specifically organized CD4 and/or MHC class II oligomers are essential for effective T cell activation.
- This provides new insights into the molecular mechanisms underlying T cell coreceptor function and immune response.
Abstract:
CD4 is a membrane glycoprotein on T lymphocytes that binds to the same peptide:major histocompatibility complex (MHC) class II molecule recognized by the antigen-specific receptor (TCR), thereby stabilizing interactions between the TCR and peptide;MHC class II complexes and promoting the localization of the src family tyrosine kinase p56lck into the receptor complex. Previous studies identified a solvent-exposed loop on the class II beta 2 domain necessary for binding to CD4 and for eliciting CD4 coreceptor activity. Here, we demonstrate that a second surface-exposed segment of class II is also critical for CD4 function. This site is in the alpha 2 domain, positioned in single class II heterodimers in such a way that it cannot simultaneously interact with the same CD4 molecule as the beta 2 site. The ability of mutations at either site to diminish CD4 function therefore indicates that specifically organized CD4 and/or MHC class II oligomers play a critical role in coreceptor-dependent T cell activation.